1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display (hereafter, referred to as LCD) suitable for a portable telephone and a portable information terminal, in which a battery is used as a drive power supply, and the like, and more particularly to a method of driving LCD so as to reduce an electric power consumption.
2. Description of the Related Art
Since the LCD is small and light and has lower electric power consumption, it is widely used in a display of a portable terminal which is represented by a portable telephone driven by a battery. For example, in the portable telephone, the LCD is used to indicate a telephone number and text of an electric mail. However, an indication of the longest time in using the portable telephone is the indication at a wait state while a call is not received. The electric power consumption in this period has enormous influence on an operation time of the portable telephone. FIG. 18 is a schematic view showing an example of the indication at the wait state. In a case of an actual portable telephone, at the wait state, a time and a calendar are indicated in general, as shown in FIG. 18. Moreover, it is necessary to indicate a remaining amount of a battery for indicating whether the portable telephone can be used or not, and also indicate a reception condition of an electric wave. Thus, the indication can not be put off even in a period except the call period (in a period at the wait state). At this time, the electric power is always consumed in the LCD. Hence, this causes the large reduction in the operation time of the portable telephone, especially, the callable time.
Even in a case of another portable terminal, it is necessary to operate the LCD if operating a calendar function and a clock function. Similarly to the portable telephone, the electric power is always consumed in the LCD. This causes the operation time of the portable terminal to be largely reduced.
In order to solve such problems, for example, Japanese Laid Open Patent Application (JP-A-Heisei, 7-230077) discloses a driving method of reducing a gradation indication voltage at a wait state. FIG. 19 is a block diagram showing the configuration of LCD similar to that disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 7-230077). This LCD is provided with a signal driver circuit 33, a liquid crystal matrix panel 34, a scanning driver circuit 35, a light source 36, a display control circuit 37, a voltage controller 38 and an interface circuit 39. The voltage controller 38 is the circuit for generating a plurality of gradation indication voltages. The signal driver circuit 33 generates a signal voltage of an amplitude corresponding to a gradation for an image data inputted through the interface circuit 39, by using the gradation indication voltage generated by the voltage controller 38, and then applies it to the liquid crystal matrix panel 34.
In the conventional LCD having the above-mentioned configuration, if the wait status is instructed through the interface circuit 39, the voltage controller 38 reduces the voltage of the gradation indication voltage to thereby reduce the electric power consumption.
In the case of the above-mentioned LCD to reduce the electric power consumption, it is possible to check the remaining amount of the battery, the current time, the reception condition of the electric wave and the like because wait state indication is displayed. However, an effective value of a voltage applied to a liquid crystal pixel is dropped. Thus, this results in a problem that the indication becomes extremely invisible when a multiple-gradation indication is displayed.
The present invention is proposed in view of the above mentioned problems. It is therefore an object of the present invention to provide a method of driving LCD, which can reduce an electric power consumption at a wait state without any deterioration in image quality.